Older thermostats are effectively a switch that closes a circuit for a low-voltage AC signal, for example 24 volts historically, that operates a relay to turn on or off the HVAC equipment. These older thermostats only required two lines, or wires, to connect and control the HVAC equipment.
Thermostats requiring power are increasingly used to control heating, ventilation and air conditioning (HVAC) equipment. These thermostats may be powered by a battery or from an externally supplied alternating current (AC) source. In thermostats powered by an AC source, at least three wires are generally required for connecting and controlling the HVAC equipment. Two wires provide a path for the AC power, while a third is used as a control signal line for controlling operation of the HVAC equipment.
In order to connect a powered thermostat requiring three wires in a location that is only provided with two wires, a ground wire adapter can be used. The ground wire adapter allows a thermostat that uses half-wave rectification of the AC signal to power the thermostat to communicate the control information to the ground wire adapter over the return wire used for the AC signal. Since the thermostat uses half-wave rectification, there would normally not be any current flowing through the return wire during a negative half-cycle of the AC signal. By causing the thermostat to conduct during the negative half-cycle, the thermostat can provide control information to the ground wire adapter in each negative half-cycle of the AC signal. The ground wire adapter can detect the signal in the negative half-cycle and control the HVAC equipment accordingly.
While the use of a ground wire adapter allows a three wire thermostat to be connected over two wires, it requires that the thermostat use only half-wave rectification, limiting the possible power consumption and efficiency of the thermostat.
It is desirable to allow a thermostat to use full wave rectification while still allowing a control signal to be multiplexed on a ground line.